Ethylene-vinyl alcohol copolymers have excellent gas barrier properties and moreover do not generate toxic gas when disposed by burning as vinyl chloride resins do. Such an ethylene-vinyl alcohol copolymer is thus widely used as a package for foods and beverages, a packing for containers, a medical infusion solution bag, a tire tube, a cushion for shoes, and the like. However, the ethylene-vinyl alcohol copolymer is a relatively hard resin and thus the flexibility is sometimes insufficient depending on the use.
There is a known method of giving various functions to such an ethylene-vinyl alcohol copolymer by containing another resin, and a variety of resin compositions containing the ethylene-vinyl alcohol copolymer and another resin are reported.
Patent Documents 1 and 2 describe a block copolymer having a polymer block mainly comprising vinyl aromatic monomer units and a polymer block mainly comprising isobutylene units, and a polymer composition containing an ethylene-vinyl alcohol-based copolymer. They describe that these polymer compositions are excellent in flexibility and rubber elasticity and also excellent in gas barrier properties. However, when the polymer compositions described in Patent Documents 1 and 2 are produced over a long period of time, the production becomes difficult due to cross-linking of the polymers, causing difficulty in stable production of the polymer composition.